Fiber optic face plates (FOFPs) are useful for the construction of liquid crystal displays with improved viewing angles. For example, U.S. Pat. No. 5,959,711 describes a benefit with the addition of one or more FOFP layers by reducing the undesirable variations in luminance, contrast ratio, and chromaticity as the display is viewed from different orientations. The viewing-angle in most LCD modes is limited due to a decrease in contrast and the presence of grayscale inversion as the display is viewed at increasingly oblique angles. As almost all commercial LCDs operate on the principle of affecting the polarization-state of light passing through them, the anisotropy in transmission-characteristics arises because the imposed phase-shift depends on the direction of propagation through the film. There are many approaches to overcome this deficiency, from the addition of passive birefringent films to designs of the pixel structure and display mode. FOFP layers in an LCD can augment or even replace many of these approaches, by azimuthally-averaging the viewing-cone.
In one embodiment, a FOFP can be located between a top polarizer and a liquid crystal layer at the front of a display device. The most common construction of a FOFP includes an array of individual glass or polymer optical fibers that are fused together with an interstitial cladding material and then cut and polished to a desired thickness to form a plate. As is well known in the art, the relevant principle of operation of the FOFP in this context is that light incident on the fiber cores at or below some acceptance angle (θMaxIN) will be transmitted through each fiber core by the process of total internal reflection (TIR) at the core-cladding interface. Known as a guided ray, no losses occur and the polar angle of the ray is preserved within the core, while the azimuthal angle of the ray is determined by the position of the entering ray and the number of reflections. As shown in FIG. 1, the consequence of this rotation is that the optical fiber 10 averages about the azimuthal position all of the incoming light entering at a given polar angle such that the output consists of a hollow exit cone 22 with a solid angle of twice the maximum entrance angle. In FIG. 1, because both illustrated incoming light rays 24, 26 enter the optical fiber 10 at an angle θMax, the solid angle of the hollow exit cone 22 is 2θMax. As the light emerging as a hollow exit cone 22 consists of an average about the azimuthal position, the transmitted light intensity is substantially equal at all azimuthal angles. It is this property of azimuthal averaging that enables FOFPs to produce symmetrical viewing characteristics over wide angles when coupled to a LCD with inherent anisotropies in luminance and contrast.
However, light incident above θMaxIN will not undergo total internal reflection, and will instead leak partially out into the cladding at every core-cladding interface. These unguided leakage rays and any light incident on the cladding itself continue to propagate through the array with no predictable azimuthal and polar angles, and are therefore scattered.
The angle θMaxIN and the angle the light beam will exit the optical fiber 10, θMaxOUT, will be the same where the relative index of refraction of the material surrounding the optical fiber 10 at the entrance plane and exit plane surfaces is the same. These quantities may differ where the material surrounding the entrance and exit plane surfaces have a different indices. Three features of the FOFP determine θMaxIN and θMaxOUT: the index of refraction of the core, of the cladding, and of the boundary material at the entrance or exit of the array. The basic relationship between these three indices and the angles is described in the following two equations, which are well known in the art:
NA=N0 sin θmax=√{square root over (N2fib−N2clad)}      θ    max    =      arcsin    ⁡          (                        1                      N            0                          ⁢                                            N              fib              2                        -                          N              clad              2                                          )      
where:
                NA=numerical aperture of FOFP        θmax=FOFP maximum solid angle of acceptance or exit        N0=refractive index of surrounding material or boundary        Nfib=refractive index of optical fiber        Nclad=refractive index of fiber cladding        
A high NA on the input face of the FOFP, through the choice of a low N0 at the input (such as air) or a large core-cladding index contrast, will lead to a wide acceptance angle. Similarly, a high NA at the output face of the FOFP will lead to a wide exit angle, which in this case is achieved by considering the N0 at the output. Conversely, a restricted θMaxIN or θMaxOUT results when a small core-cladding index contrast is chosen or when a high index material is used as the surrounding material, such as plastic, polyamide, or optical glass.
A FOFP could also be located at the rear of a display device, especially a display device with a rear illumination source. For example, a rear FOFP could be positioned between a diffuser and a liquid crystal cell. The rear FOFP may include an input face, facing and adjacent to the diffuser, which has a high θMaxIN resulting in a relatively wide light acceptance angle. An output face of the rear FOFP is opposite the input face, where the output face provides a low θMaxOUT. Therefore the FOFP has a relatively narrow light exit or output angle. The rear FOFP provides increased collection of light from the rear illumination source, thereby providing an improvement in the luminous efficiency of the LCD.
FIG. 2 illustrates a top view of a FOFP 28 made of an array of individual optical fibers that are fused together with an interstitial cladding material and then cut and polished to a desired thickness to form a plate. The core 12 and cladding material 14 can be seen on the surface of the FOFP 28. FIG. 3 illustrates a closer view of a portion of the FOFP 28 of FIG. 2.